1. Field of the Invention
The present invention generally relates to methods and molds for casting molten metal articles of variable thickness and more specifically to such molds having heating and cooling features for controlling the cooling rate of portions of the cast article within the mold during casting.
2. Description of the Prior Art
When casting metal articles having thick and thin sections, it is desirable to have these sections cool in an equalized manner in order to minimize residual stresses and to produce a more sound casting. However, equalized cooling is difficult to achieve using conventional casting molds and processes since the thin sections of the casting naturally tend to cool at a faster rate than the thicker sections.
A casting mold and process has been recently developed in the United States for achieving such results. The inventor of this mold and process is Raymond H. Witt and the mold and process are disclosed in two related U.S. Pat. Nos., namely 4,742,863 granted May 10, 1988 and 4,674,553 granted Jun. 23, 1987. Witt's contribution to the metal casting art was to provide a permanent metal type mold having an oversized casting cavity lined with a variable thickness sand liner. In order to achieve equalized cooling of the thick and thin sections of the casting, the thickness of the sand liner is varied inversely with the thickness of the casting such that the sand is thicker in sections where the casting is thin and thinner in sections where the casting is thicker. This effectively increases the cooling rates of the thick sections while at the same time decreases the cooling rates of the thinner sections. By adjusting the cooling rates of these sections, their respective cooling times can be equalized. Very high quality, sound castings have been produced this way.
Although the above mold and process has shown to work well for equalizing the cooling times of thick and thin sections of a casting in most all applications, there are still other applications where it is desirable to vary the results achieved by the above mold and process. Specifically, there are some applications in which sections of the casting are very thin and a sand liner (no matter how thick) cannot lower the cooling rate of these sections sufficiently to achieve equalized cooling with the other sections of the casting.
In still other applications, it is desirable to harden the surfaces of certain sections of the casting. Usually, this is achieved through a subsequent heat treating operation following casting. However, surface hardening can also be achieved by rapidly chilling the surface during casting. Solid metal chills are typically employed for this purpose. Examples of such solid metal chills are disclosed in U.S. Pat. Nos. 936,623 to Griffith, granted Oct. 12, 1909; 1,524,391 to Durham, granted Jan. 27, 1925 and 1,876,073 to Player, granted Sep. 6, 1932.
A problem arises, however, when the surface to be hardened is a very thick section of the casting. In this case, conventional solid metal chills are inadequate since they are incapable of extracting heat from the surface of the casting at a fast enough rate to develop the desired hardened properties. This is so even when used with the thick and thin type casting molds.